1. Field of the Invention
This invention relates to a dry sump type lubrication device for a motorcycle with an oil tank disposed in front of a crankcase.
2. Description of Related Art
Recent years, motorcycles for motocross races have made progress in the changeover of their engines to four-stroke systems. Since the motorcycle of this type runs on rough ground of great irregularities, it is necessary to raise the mounting position of the engine to secure a sufficient minimum road clearance. Therefore, a dry sump type four-stroke engine is preferably used without an effective oil reservoir in the bottom of the crank chamber.
The dry sump type four-stroke engine is provided with an oil tank independent from the crankcase, and lubricating oil is forcedly circulated between the oil tank and the crankcase. Specifically, lubricating oil returned to the crankcase after lubrication of engine parts is sent into the oil tank through a first oil pump, and lubricating oil introduced from the oil tank to the crankcase is fed to the engine parts through a second oil pump. The first and second oil pumps are assembled in the bottom of the crankcase and connected to the oil tank through a connecting device such as pipes or hoses, respectively.
However, the oil tank has some volume and it is desirable that the oil tank is disposed as low as possible taking into account of lowering the center of gravity of the motorcycle. As a system to meet this requirement, a motorcycle has been known in which an oil tank is disposed right in front of the crankcase. This motorcycle is provided with a cradle type frame having left and right down tubes running around the crankcase from the front thereof toward a region therebelow, and an oil tank is disposed in a region surrounded by these down tubes and the forward end of the crankcase.
The conventional oil tank discussed above secures its volume by elongating its lower end such that it extends into a region below the crankcase. However, in this arrangement, since the oil tank protrudes downwardly from the crankcase, the minimum road clearance is lowered by as much size as the oil tank.
As a result, in a motorcycle for motocross races in particular, the oil tank might strike against the road surface during running. Therefore, in a motorcycle of an off-road model intended to run on rough ground, it is necessary to form the oil tank as compact as possible to decrease it downward protrusion from the crankcase.
However, an oil tank with the size reduced results in a decreased tank volume and a new problem arises as described below.
In a typical dry sump type four-stroke engine, the first oil pump for collecting lubricating oil returned to the crankcase has a larger volume than the second oil pump for feeding lubricating oil. Therefore, in a condition in which engine speed is low during an idling operation, for example, lubricating oil returning to the oil tank and lubricating oil drawn from the oil tank become ill-balanced and the inside of the oil tank is filled completely with lubricating oil.
In other words, if the amount of lubricating oil returning to the oil tank becomes larger than the amount of lubricating oil drawn from the oil tank, the change in oil quantity cannot be absorbed within the oil tank. Therefore, the inside pressure of the oil tank rises excessively, which may cause damage of the oil tank or leakage of lubricating oil from the connecting portions between the oil tank and connecting device.
An advantage of this invention is to provide a dry sump type lubrication device for a motorcycle capable of preventing excessive rise of the inside tank pressure while effecting size reduction of the oil tank.